1. Field of the Invention
This invention relates to an improved process for the recovery of phosphoric acid from phosphatic-mineral-containing material; and more particularly relates to, in contrast to conventional sulfuric acid wet-processes, obtaining a more concentrated and/or higher purity phosphoric acid from an intermediate precipitated dicalcium phosphate as well as avoiding losses of P.sub.2 O.sub.5. In this specification phosphate values, except where indicated to the contrary, will be expressed on a weight percent basis and as phosphoric acid anhydrite (P.sub.2 O.sub.5) rather than as for example expressing phosphate content of phosphoric acid as percentage H.sub.3 PO.sub.4. Thus, for illustration, a reference to phosphoric acid solution as having a 30-34% P.sub.2 O.sub.5 phosphate content corresponds to a concentration of 40-47% expressed as H.sub.3 PO.sub.4.
The present invention also relates to a wet chemical process for directly forming phosphoric acid of higher purity and/or greater concentration involving a different sequence of steps without the necessity of evaporation.
The process of the present invention is an improvement over the usual wet processes for the production of phosphoric acid from natural phosphate rock with the aid of concentrated sulfuric acid, wherein the phosphate rock, ground to a suitable fineness, is digested slowly, fed by a fairly concentrated sulfuric acid (60-96% by weight), with the formation of calcium sulfate in finely divided condition. In those processes the time of digestion with the sulfuric acid is about 4 hours; and the usual maximum concentration of the resultant phosphoric acid without evaporative concentration is from 40% to 50% H.sub.3 PO.sub.4 (or about 28% to 37% P.sub.2 O.sub.5). This high a concentration requires a very high quality and high purity phosphate mineral, i.e. about 80 B.P.L., to minimize impurities build up and consequent viscidity and sludges in the obtained phosphoric acid. The calcium sulfate which is formed in such a process entrains large quantities of phosphoric acid, the recovery of which entails voluminous washings with water with consequent dilution of these portions of the phosphoric acid product to an extent such that subsequent concentration by evaporation is necessary to produce phosphoric acids suitable for most purposes and which may be transported economically. Because of its corrosive nature and the build up of sludges, concentration of phosphoric acid by evaporation is a difficult and expensive operation. Efforts to increase the concentration other than by evaporation of the acid resulting from the initial digestion of phosphate rock with sulfuric acid in the wet process have resulted in incomplete decomposition of the phosphate rock and the production of a more finely divided calcium sulfate that is more difficult to settle and filter.
Concerning the material suitable for use in the typical wet processes, generally beneficiated rock is required and it should have a content of about 64-87 B.P.L., or about 29-40% P.sub.2 O.sub.5 content. Many ores have calcium carbonate associated with the phosphate that cannot be removed by beneficiation, and its quantity is detrimental to sulfuric acid consumption. Further carbonate contributes to foaming during acidulation. Organic matter present in the rock tends to decrease filtration rates and reduce porosity of any filter cakes obtained through hindering well developed gypsum crystal growth. Thus it is common to require the additional step of calcining the rock before use to reduce the organics in the carbonate. Of even greater importance in treating the rock, residual iron and aluminum, hereinafter "I & A", may interfere with the growth of the gypsum crystals in the acid production; cause sludges to form in the resultant acid; and cause a form of insoluble and unavailable phosphates to form in further products made of the resultant acid. Even after the resultant acid has been clarified, filtered and vacuum evaporation concentrated, wet process phosphoric acid deposits scale and sludge with time. The normal clarification over a several day period of time results in the separation of a sludge that contains about 40% P.sub.2 O.sub.5 which requires further processing to avoid considerable phosphate value loss; and such further processing of the sludge is difficult. In addition many ore deposits contain considerable material of much lower phosphate value than can be economically utilized.
Generally in the improved wet processes of the art, beneficiated rock is dissolved in a mixture of fresh sulfuric acid and recycled phosphoric acid, obtained in the process, followed by filtration removal of the gypsum precipitated. This results in a filter acid of about 29-32% P.sub.2 O.sub.5 which may be then concentrated to higher P.sub.2 O.sub.5 value by vacuum evaporation which is quite energy intensive. The vacuum evaporation must be carefully controlled due to circulating nuclei of impurities, that is, impure calcium sulfate, fluorides and silicates, and to avoid their formation as scale on the equipment. In the digesting of the rock with the strong acid, about 97% of the sulfate is converted generally to gypsum and some 2-3% appears to remain in the phosphoric acid as dissolved sulfate. That dissolved sulfate content must be closely controlled to avoid calcium sulfate scaling at a later stage.
Regarding the resultant gypsum, this is considered as a practical manner as an unuseable by-product which must be disposed of. Phosphoric acid tends to become occluded in the gypsum crystal nuclei; and the shape and manner of gypsum growth tends to in many cases capture more valuable phosphate value and can end up in a virtually unfilterable mass that is difficult to handle and dispose of. In addition fluorosilicates and fluoro-aluminates can complex on the growing gypsum crystal, depending upon the sulfuric acid concentration present, which also deleteriously affects gypsum crystal shape and size. Further if small unfiltered gypsum crystals remain in the recycled phosphoric acid they tend to, at normal operating temperatures, re-dissolve and then re-precipitate on the phosphate values of the rock before the acid can digest the phosphate, thus also contributing to further phosphate losses in the system. Thus phosphate losses attributable to the gypsum include encapsulated unreacted phosphate containing material, entrapped liquid and a third type, where some HPO.sub.4.sup.-2 ions replace some of the SO.sub.4.sup.-2 ions in the growing crystal structure, sometimes referred to as syncrystallized phosphate gypsum. All of these phosphate losses will hereinafter be referred to as "occluded phosphate".
2. Description of the Prior Art
Virtually all of the efforts in the art of chemically leaching phosphate values from phosphate-mineral-containing materials appear to have been directed to acidulating the ore or rock directly to phosphoric acid solutions without an intermediary precipitated salt; and these efforts involve the use of strong leaching solutions in concentrated forms to obtain directly a strong leach liquor which is the virtual end product of such processes. However, there has been some incidental suggestion of some sort of intermediary precipitated salt material in a few of the art teachings. For example, U.S. Pat. No. 2,013,970 to Moore shows an intermediate formation of dicalcium phosphate in a concentrated sulfuric acid leaching of ground phosphate rock. Preferably, according to this process, the intermediary is then calcined to calcium pyrophosphate and treated with pure sulfuric acid to obtain a 45.degree.-50.degree. Baume phosphoric acid (about 44-51% P.sub.2 O.sub.5). However in a non-preferred embodiment it is suggested that the dicalcium phosphate cake, if sufficiently pure, might be treated with concentrated sulfuric acid to convert the phosphate to ortho-phosphoric acid without the preferred calcination. It is noted that in the initial leaching the phosphatic material is subjected to the action of sufficient sulfuric acid to form a mono-calcium phosphate solution but insufficient to form more than a minor proportion of free phosphoric acid in the leach liquor; and it is the precipitated product from this reaction that is being utilized by Moore for subsequent treatment. Secondly, the teachings on conversion of the dicalcium phosphate intermediary to ortho-phosphoric acid without the calcination steps would appear to, due to the concentrations of the materials being dealt with and the lack of dispersion of ionic forces in reaction, cause an encapsulation of a substantial part of the dicalcium phosphate particle being dissolved by the gypsum being precipitated thus making it difficult to completely get at, extract and dissolve the phosphate values and also result in entrapping large quantities of phosphate values with the gypsum being precipitated. Further it is noted that in this ostensible variation the phosphoric acid is leached out with previously prepared 25.degree.-30.degree. Baume phosphoric acid after the sulfuric acid treatment and gypsum precipitation, which offers additional obvious disadvantages. This patent ostensibly is an improvement over Moore's prior U.S. Pat. No. 1,910,808, in which he attempted to remove impurities before conversion of the leached phosphate content to phosphoric acid by converting the mono-calcium phosphate solution successively to ammonium phosphate, and then a "water insoluble calcium phosphate" and finally phosphoric acid; and his prior U.S. Pat. No. 2,233,956.
Another proposed approach has been to form as the starting reactant a complex or polyphosphate which is then digested with either sequentially concentrated phosphoric acid and then sulfuric acid or with a mixture of concentrated sulfuric acid phosphoric acids to result in ortho-phosphoric acid of high concentration (U.S. Pat. Nos. 2,338,408; 2,384,813; and 2,384,814). However such proposed processes require an energy intensive heating to form the complex or polyphosphate to avoid impurities, particularly fluorine and I & A, being carried through the process and to avoid crude, low grade and low phosphorous content resultant acid product.
Despite patent literature statements to the contrary on purity and filterability of co-produced gypsum; this material essentially remains a difficult to handle and drain, slowly filtering media which as a practical matter traps too much phosphate value and generally results in a sludge frequently causing process stoppages. Much of the older patent literature does not discuss how much phosphate value, as P.sub.2 O.sub.5, is occluded or what the actual filtering rates are in their processes. However, as a practical matter, refinements to the old wet processes still leave about generally 2-3% phosphate values as P.sub.2 O.sub.5 occluded in the gypsum; filtration is difficult and the calcium sulfate dihydrate is contaminated with considerable sands, fluorine silicates and the like.